1. Field of the Invention
The present invention relates to a one-piece golf ball, more particularly to a technology for improving the resilience and durability of a rubber composition used for the one-piece golf ball.
2. Description of the Related Art
The one-piece golf ball consisting of a single layer is generally a spherical molded body of a rubber composition containing a base rubber, a co-crosslinking agent and a crosslinking agent. The characteristics of a golf ball such as resilience and durability are highly affected by the rubber composition constituting the one-piece golf ball. For example, the high resilience provides the golf ball with the long distance when hit, and the high durability suppresses the generation of cracks when the golf ball was used repeatedly. Since the one-piece golf ball consists of 100% rubber composition, the one-piece golf ball is affected more significantly by a rubber composition, especially a base rubber contained in the rubber composition, compared with multi-piece golf balls such as a two-piece golf ball and a three-piece golf ball. Thus the improvement of the rubber composition has been under investigation in order to improve the characteristics of the golf ball.
For example, Japanese patent publication No. 2002-282393A discloses a polybutadiene with 99% or more of cis-1,4-bond synthesized using a catalyst consisting of a composition which comprises a metallocene catalyst of a rare-earth metal compound, an ionic compound consisting of an uncoordinated anion and a cation, and/or aluminoxane. Japanese patent publication No. 2002-338737A discloses a polybutadiene with 80% or more of cis-1,4-bond having a molecular weight distribution (Mw/Mn) of 1.2 to 2.0 synthesized using a catalyst consisting of a composition which comprises a metallocene catalyst of a rare-earth metal compound, an ionic compound consisting of an uncoordinated anion and a cation, and/or aluminoxane. Additionally, Japanese patent publication No. 2002-35167A discloses a one-piece golf ball consisting of the rubber composition containing two-kinds of the polybutadienes having a specific molecular weight in a specific mixing ratio.
However, in a technology wherein the metallocene catalyst is used to enhance the content of cis-1,4-bond, there exists a problem that it is unsuitable for industrial use since the metallocene catalyst is unstable in the air. In addition, the resilience and durability of the golf ball is not improved sufficiently by merely adjusting the molecular weight of the polybutadiene rubber, and thus there is a need for the further improvement. Further, the polybutadiene has bonding types such as trans-1,4-bond and 1,2-vinyl bond other than cis-1,4-bond, and it is known that the high content of trans-1,4-bond before the crosslinking lowers the resilience.